Energy changes and reversible reactions
● Energy is the ability to do work or produce heat
● During a reaction, there is always an energy change
○ Exothermic → gives out energy / temperature rises
○ Endothermic → takes in energy / temperature falls
● Reactants → products + energy
● Reactions + energy → products
● Exothermic
○ If the energy taken in to break bonds is less than the energy released in making bonds,
the reaction is exothermic
● Endothermic
○ If the energy taken in to break bonds is greater than the energy released in making bonds,
the reaction is endothermic
● Bond energy
○ The bond energy is the energy needed to break bonds or release them when these bonds
form. It is given by kJ/mol
○ The amount of energy absorbed in breaking of covalent bonds is called bond energy. Same
energy is released when the bonds form
Bond energy
H - H
436 kJ
Cl - Cl
242 kJ
H - Cl
431 kJ
C - C
346 kJ
C = C
612 kJ
C - O
358 kJ
C - H
413 kJ
O = O
498 kJ
O - H
464 kJ
N ≡ N
946 kJ
N - H
391 kJ
Energy level diagram
● Exothermic
○ An energy level diagram for an exothermic reaction, THe products have lower energy then
the reactants
● Endothermic
○ An energy level diagram for an endothermic reaction. The products have higher energy
than the
reactants
Bond energy
● Endothermic reaction
○ 2NH3 ⇌ N2 + 3H2 [Haber-Bosch process]
= 2[(N - H)] x 3] ⇌ N ≡ N + 3(H - H)
= 2[(391) x 3] ⇌ [946] + 3[436]
= 2346 kJ ⇌ 2254 kJ
Total energy (ΔH) → 2346 - 2254
= +92 kJ/mol
○ If answer is in positive then it has to be an endothermic reaction
● Exothermic reaction
○ H2 + Cl2 ⇌ 2HCl
(H - H) + (Cl - Cl) ⇌ 2 (H - Cl)
678 kJ ⇌ 862 kJ
○ Total energy(ΔH) → 678 - 862
= -184 kJ/mol
○ If the answer is in negative then it has to ve an exothermic reaction
● Reactants are at a lower level
than the products
● More energy is used to break
bonds than to make them
● In this case, the ΔH is positive
therefore it is an endothermic
reaction
● Reactants are at a higher level
● More energy is used to make bonds than to break
. those bonds.
● In this case, ΔH is negative therefore this is an
. exothermic reaction
● Activation energy arrow always has to be double edged (⇌)
● Delta H arrow has to be single edged (-->)
● Examples of exothermic reactions
○ Neutralization reaction
○ Respiration (C6H12O6 + 6O2 → 6CO2 +
6H2O)
○ Combustion
○ Rusting
● A fuel is a substance we use to provide energy
● Good fuel
○ More heat per tonne of fuel
○ Less pollution
○ Steady and reliable supply
○ Should be easy and safe to store and
transport
● Burning fuel is exothermic, the more heat/energy
given out, the better
● Equilibrium
○ Rate of forward reaction is equal to the rate of backward reaction
○ No overall change (concentrations remain the same)
● Dynamic equilibrium
○ Continuous change
○ Ammonia breaks down to form N2/ H2 and the N2 and H2 combine to make more ammonia,
therefore its a continuous change
● Closed system
○ The reversible reaction reaches a state of dynamic equilibrium, where the forward and
backward reaction take place at the same rate.
○ All conditions remain the same (temp/pressure/catalyst)
○ There is no overall change
● Le Chatelier Principle
○ Any change made to a closed system
○ The system will try and oppose the change and restore equilibrium
○ WHen a reversible reaction is in equilibrium and you make a change, the system acts to
oppose the change and restore equilibrium
Factors that affect yield
● Change in temperature
○ In Haber process, the forward reaction is exothermic so the reverse reaction is
endothermic
○ When temperature is increased, the position of the equilibrium moves in the endothermic
direction as to reduce the temperature therefore the position of the equilibrium moves to
the left and reduces yield
○ If temperature is too low, then it takes too long to produce the yield and time is money so
therefore moderate temperatures are kept
● Change in pressure
○ An increase in pressure will cause the equilibrium to move to the side with the fewest
molecules of gas to reduce pressure. The position movies to the right therefore the yield
increases and vice versa
○ Equilibrium shifts in the direction that produces the smaller number of molecules of gas to
decrease pressure
○ A change in pressure will shift the equilibrium only if the number of molecules if different
on
each side of the equation
○ A change in pressure will shift the equilibrium only if the reactants and the products are
gases
● Adding catalyst
○ A catalyst speeds up the forward and backward reaction equally.
○ Equilibrium is achieved faster which saves time but amount of yield produced does not
change
● Renewable reaction in solutions
○ By adding more of a reactant (increasing concentration). So more product will form to
oppose this change
○ By changing the temperature, a rise in temperature will favour endothermic reaction
Optimum conditions for Haber process
● Temperature → 450 degree celsius
○ Faster rate of reaction
○ Results in low yield but can be done multiple times therefore eventually economically viable
○ Too low temperature can make reaction slow and make it not economic
● Pressure → 200 atmp
○ Equilibrium will move towards side with lesser particles (ammonia)
○ High pressure = higher yield and faster rate of reaction
○ Too high pressure = chances of explosion, too costly to maintain
○ 4 particles of reactant gas becomes 2 particles of product gas
● Iron catalyst
○ Speeds up reaction
○ Iron catalyst allows a lower temperature to be used (saving energy, higher yield)
○ Iron catalyst increases rate of reaction equally in both reactions (exothermic and
endothermic)
Chemical tests for water
● Anhydrous copper(II) sulfate
○ Pure copper(II) sulfate is white and is also known as anhydrous copper(II) sulfate as it has
no water in it
○ When water is present, the Anhydrous copper(II) sulfate turns blue (the colour change
from white to blue to used to detect the presence of water)
○ CuSo4 + 5H2O ⇌ CuSo4 . 5H2O
(s) (l) (s)
------------------ ---------------
White blue
● Blue Anhydrous cobalt(II) chloride
○ It turns pink in the presence of water
○ [Co(H20)]2+ + 4Cl- ⇌ [CoCl4]2- + 6H2O
-------------- ------------
Pink blue
